The present invention relates to a boiling refrigerant type cooling system preferably applicable to cooling of a heat generating body such as a semiconductor device, and more particularly, to a boiling refrigerant type cooling system to realize stable start of boiling even with respect to a heat generating body with high heat generation density.
In recent years, a semiconductor device such as a central processing unit (CPU) incorporated in electronic devices such as a personal computer or a server has an increasing heat release value due to miniaturization and high integration. However, in the above-described semiconductor device, generally, its performance cannot be maintained at a temperature equal to or higher than a predetermined temperature, and further, the device may be broken in some cases. Accordingly, temperature management by cooling or the like is required. Generally, cooling is realized with an air cooling system using a heat sink and a fan to send air to the heat sink. However, localization of heat generating part occurs due to the above-described miniaturization and high integration. Further, reduction of electric power consumption in the electronic devices and noise reduction are required in recent ecology-conscious society.
From the above-described object and requirements, there is a strong request for an efficient cooling technology such as a liquid cooling system using a refrigerant such as water in place of conventional air cooling systems. Among the above-described liquid cooling systems, a boiling refrigerant type cooling system especially attracts attention. This system utilizes liquid boiling and latent heat of vaporization of the refrigerant to obtain high cooling efficiency.
Note that as a conventional technique related to the present invention, e.g., Patent Japanese Published Unexamined Patent Application Nos. 2011-47616, 2001-77256 and 2008-147482 disclose a cooling system, having a heat reception jacket which is thermally connected to a heat-generating semiconductor device or the like and which boils a contained liquid refrigerant, and a condenser which receives refrigerant steam from the heat reception jacket and condenses the steam to the liquid, to circulate the refrigerant by phase change.
Further, Japanese Published Unexamined Patent Application No. 2005-164126 discloses a technique to provide minute tunnels communicating with the outside via minute holes or gaps under a surface in contact with liquid on a boiling surface, to improve heat transfer performance of the boiling surface.
When a heat generating body is cooled using the above-described boiling refrigerant type cooling system, there is a problem of overshoot which occurs especially upon start of boiling. That is, as shown with a broken line in FIG. 1, the temperature of the boiling surface is increased to the start of boiling, then radically reduced after the start of boiling, and stabilized. The difference between the maximum temperature upon start of boiling and the temperature in stable boiling is overshoot. The overshoot occurs due to a similar phenomenon to bumping (explosive boiling phenomenon).
Even when heat generation and stoppage of heat generation of a heat generating body are repeated under the same external conditions, the level of overshoot and time from the start of heat generation to the start of boiling differ each time as shown in FIG. 2. Further, in some cases, boiling does not start even when several hours have elapsed from the start of heat generation. Accordingly, when a semiconductor device or the like is cooled using a boiling refrigerant type cooling system, it is necessary to completely suppress the above-described overshoot. When the overshoot occurs, there is a probability that the temperature is raised beyond a permissible limit of the semiconductor device to cause failure or breakage of the semiconductor device.
The above-described Patent Japanese Published Unexamined Patent Application Nos. 2011-47616, 2001-77256 and 2008-147482 disclose a technique to improve heat transfer performance by boiling, although they do not disclose a technique to suppress the overshoot.
To suppress the overshoot, it is necessary to provide a structure to promote generation of initial bubbles as a start of boiling upon start of heat generation. That is, it is necessary to suppress the occurrence of overshoot by inducing the generation of bubbles.
According to the structure in Japanese Published Unexamined Patent Application No. 2005-164126 shown in FIG. 3, when bubbles as a start of boiling occur, then the bubbles spread in the tunnels, and the tunnels are filled with a steam layer. Then as shown in FIG. 4, the steam layer comes into contact with the liquid through the minute holes or gaps, to realize continuous boiling.
However, when Japanese Published Unexamined Patent Application No. 2005-164126 is formed on a plane surface, as the respective tunnels are provided in parallel as shown in FIG. 5, when one bubble is generated somewhere, the steam layer is formed in merely one tunnel. Accordingly, to spread the steam layer over the entire boiling surface, it is necessary to cause many bubbles. However, as the bubbles are generated at random, it is impossible to suppress the unstableness upon start of boiling as shown in FIG. 2 with the structure in Japanese Published Unexamined Patent Application No. 2005-164126.
Further, to suppress the overshoot, it is necessary to provide a structure to prevent percolation of foreign material into the boiling surface, especially percolation of brazing material used in hard soldering into the boiling surface. Generally, the characteristic of boiling phenomenon is much influenced by wettability between the liquid and the material of the boiling surface, i.e., the contact angle. Generally, it is known that the boiling heat transfer performance is improved when the contact angle is small. When the contact angle is small, the liquid flows between the bubbles and the boiling surface, to reduce the respective bubbles and the steam layer having a low thermal conductivity is thinned. On the other hand, upon generation of initial bubbles as a start of boiling, as bubble growth is disturbed with the flow-in of the liquid when the contact angle is small, the start of boiling is stable when the contact angle is large. Accordingly, the percolation of foreign material such as brazing material with a smaller contact angle than that of the material of the boiling surface into the boiling surface becomes a factor of unstableness of start of boiling.